The present invention relates to organic semiconductor components, and to their production.
Semiconductor layers employed in the organic semiconductor components sector are primarily amorphous. The lack of order in these amorphous layers is a disadvantage for a variety of physical properties, for example for the important conductivity of the semiconductor layers. A very specific disadvantage for the efficiency of the components arises, however, in the area of the light-emitting components, more particularly of organic light-emitting diodes. In these components, the unoriented emission harbors a large loss factor for the external quantum efficiency, i.e., the fraction of photons generated that is also actually emitted to the outside. Existing organic light-emitting diodes feature an external quantum efficiency, without outcoupling aids, of not more than about 20%.
The efficiency of organic light-emitting diodes is measured using the light yield. Besides the internal quantum efficiency, which is determined by parameters inherent in the material of the emitters and by the self-absorption properties of the semiconductor layers, optical parameters make a large contribution to a reduction in the external quantum efficiency—that is, the photons actually emitted to the outside. These parameters are, for example, incoupling losses into the glass substrate, the excitation of waveguide modes, and the losses due to excitation of plasmons in the reflecting electrodes. In order to date to minimize the losses due to undirected emission within the OLED, the reflective electrodes have been fabricated from reflective material such as aluminum or silver, for example, leading to high reflection of the photons generated. This solution, however, is not very effective, since the excitation of plasmons in the electrodes means that there are again large losses of the generated light quanta here also. These losses due to plasmons amount to approximately 30%. These losses can only be reduced if a lower proportion of the photons generated actually impinges on the reflective electrodes to start with. In other words, the emission would have to be directed in such a way that the number of emitting dipole vectors normal to the reflective electrodes becomes minimal.
A fundamental barrier to the orientation of the emitters, however, is that first of all it is necessary to know the direction in which a molecule is emitting, in relation to its internal molecular coordinate system. Depending on the spatial orientation of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), the first excited state has a different dipole moment from the ground state. The emission dipole correlates with the dipole moment in the ground state.